1. Field of the Invention
This invention relates to methods of determining residual oil saturation in watered out oil reservoirs, and more particularly relates to a method of using a partitioning tracer and a non-partitioning tracer in combination with logging techniques to obtain information used to determine residual oil saturation in a reservoir.
The determination of the amount of oil in subterranean reservoirs has been a subject of considerable interest to the oil industry for many years. Such information is obviously of interest in the case of a newly discovered reservoir which is to be produced, and is of even greater interest in the case of a reservoir which has been subjected to primary production and possibly secondary production. In recent years the increased interest in recovering additional quantities of oil from reservoirs which have been subjected to primary and secondary production, by enhanced recovery methods such as surfactant flooding and the like, has increased. Since such enhanced oil recovery methods are quite expensive, it is important that a method be available for determining the amount of residual oil in a formation to be subjected to enhanced recovery prior to the initiation of the enhanced recovery operation. The residual oil present in reservoirs which have been subjected to primary or secondary recovery is oil which remains in place after waterflooding operations, or after water flushing by a strong water drive or the like. Such residual oil is trapped in the formation pores or otherwise retained in the formation by various mechanisms many of which are only partially understood at the present. In any event, such oil while recoverable in many instances by enhanced recovery operations is not recoverable by natural water drive or by conventional waterflooding operations.
As a result of the increasing interest in the recovery of additional quantities of oil from reservoirs which have been subjected to waterflooding operations, the oil industry has directed a continuing effort to the development of improved methods for determining the amount of residual oil in such formations.
The present invention provides a method for obtaining information useful in determining the residual oil saturation in a reservoir.
2. Prior Art
There are several methods which are currently used to obtain the fluid saturations of a formation. Coring of the reservoir is one commonly used technique for acquiring this information. Coring is accomplished by drilling into the reservoir and obtaining a sample of the formation rock, saturated with its fluids, which is cut from the formation and removed to the surface where its fluid content can be analyzed. This method, however, is susceptible to faults of the sampling technique; thus, the sample taken may not be representative of the formation as a whole. Also, there is a high likelihood that the coring process itself may change the fluid saturation of the extracted core. Moreover, coring can only be employed in newly drilled wells or open hole completions. In the vast majority of wells, casing is set through the oil-bearing formation when the well is initially completed. Core samples, therefore, cannot be subsequently obtained from such a well. Finally, coring by its very nature only investigates the properties of the formation rock and fluids in the core itself, and this data cannot always be extrapolated with accuracy to areas of the formation removed from the well.
Another approach for obtaining reservoir fluid saturations is by interpretation of well logs. While this technique is relatively simple and inexpensive to carry out, logging investigates the formation for only a short distance beyond the well bore. Moreover, the logs are a measure of the properties of the rock-fluid system as an entity, and it is difficult by this approach to differentiate between the properties of the rock and its fluids.
Fluid saturations can also be approximated by material balance calculations based on production histories. However, estimates of fluid saturations made by this method are subject to even greater error than coring or logging since the initial fluid saturation must be determined by some other technique and an accurate history of the quantity and source of the produced fluids independently obtained.
U.S. Pat. No. 3,623,842 to Deans proposes a method for measuring fluid saturations in a hydrocarbon-bearing formation containing a mobile fluid and an immobile fluid. In that method, a carrier liquid containing a reactant tracer material is injected into the formation and displaced away from the well. The reactant material is permitted to remain at rest in the formation for a period of time. During this "soak period" at least a part of the reactant reacts to form a product having a different partition coefficient between the carrier fluid and the immobile phase than the reactant material. Subsequently, the carrier fluid, the unconsumed reactant and the reaction product are displaced through the formation, preferably to the injection well by produced fluids. Since the reactant and the product have differing partition coefficients between the mobile and the immobile phases, they are chromatographically retarded in their passage through the formation by different amounts which are a function of the saturation of the immobile fluid phase. By detecting the presence of the reactant and the product in the produced fluids and analyzing these results by chromatographic techniques, the relative proportions of mobile and immobile fluids in the formation can be determined.
While the proposed reacting tracer method overcomes some of the disadvantages of the other known methods for determining fluid saturations, other disadvantages are encountered which adversely affect the accuracy and utility of this method. For example, where the unreacted material is employed as one of the tracers, the method requires the use of a reactant material that is only partially reacted under the reservoir conditions so that both the reactant material and the product are present in the produced fluids in detectable quantities. It is difficult to select a reactant material that exhibits a desired partition coefficient, that only partially reacts under a wide variety of reservoir conditions to form a product having a suitable, but different partition coefficient, and in which both the reactant and the product are present in detectable quantities in the recovered fluids. In order that detectable concentrations of both the reactant and the product materials be obtained, it is often necessary to increase the concentration of the reactant material in the injected carrier liquid. Also, the residence time that the tracer materials are in the reservoir must be carefully controlled to obtain sufficient reaction to provide a detectable concentration of product, yet retain detectable concentrations of reactant. This is difficult and costly to carry out because of the increased chemical usage, difficulty in finding suitable tracers, difficulty in predicting reaction conditions, and because of the careful control of residence or exposure time that is required. Thus, need exists for a simplified method for obtaining reservoir information which can be used to determine fluid saturations in a petroleum reservoir.
U.S. Pat. No. 4,071,756 describes a method for determining residual oil saturation utilizing programmed injection of a first water soluble radioactive tracer and a second partitioning tracer in conjunction with known logging techniques.
While the procedures described in the above-discussed references have been useful to an extent, there has been a continuing need for improved methods of obtaining data which enables the determination of residual oil saturation in a watered out reservoir.